1. Field of the Invention
The present invention relates to a regenerating process to reuse for an SOI layer wafer a layer transferred wafer obtained as a by-product in a so-called ion implantation separation method (which is also referred to as a smart-cut method) wherein an ion implanted wafer is separated after bonding to manufacture SOI (silicon on insulator). In particular, the present invention relates to a process for regenerating a layer transferred wafer many times.
2. Description of Related Art
As a manufacturing process of an SOI wafer, generally, there has been conventionally known an ion implantation separation method wherein an SOI layer wafer consisting of single-crystal silicon having a hydrogen ion or a rare gas ion implanted therein is bonded to a support wafer through an oxide film and separated in an ion implanted region by a heat treatment in order to manufacture an SOI wafer. A cleaved surface of the SOI wafer obtained by this process is an excellent mirror surface, and hence the SOI wafer can be obtained of which film thickness has high uniformity.
Further, as a process for obtaining as a by-product a layer transferred wafer in a lower portion of the SOI layer wafer which has not been used for an SOI layer of the SOI wafer and reusing this layer transferred wafer for an SOI layer wafer in order to remanufacture an SOI wafer, there are disclosed a process for regenerating a layer transferred wafer and a layer transferred wafer (see, e.g., Patent Reference 1).
In this process for regenerating a layer transferred wafer, when an ion implanted area at least in a chamfered portion of a layer transferred wafer is removed, and then a surface of the layer transferred wafer is polished to be reused for an SOI layer wafer, the ion implanted area does not remain in the chamfered portion even if a heat treatment is performed. Therefore, the chamfered portion is not separated and no particle is produced, and hence no particles adhere to a device region of an SOI wafer. Further, the SOI wafer manufactured by reusing the layer transferred wafer for an SOI layer wafer has a high quality and an excellent yield ratio.
[Patent Reference 1] Japanese Unexamined Patent Application No. 2001-155978 (claim 2, claim 4, paragraphs [0012], [0015] and [0017], FIG. 4)
However, the above-mentioned conventional process for regenerating a layer transferred wafer is characterized in that a layer transferred wafer is subjected to a heat treatment and then a surface of the layer transferred wafer is polished (See Patent Reference 1 [claim 4]). The heat treatment conditions before polishing thereabove, the layer transferred wafer is held in an oxidizing atmosphere at a temperature of 500° C. or higher for several minutes to several hours. Alternatively, the layer transferred wafer is held at 1000° C. for 30 minutes in Second Embodiment thereabove. Here, in the heat treatment of holding at the heat treatment temperature of 1000° C. for 30 minutes, generation of oxygen precipitates or oxygen precipitate nuclei in the layer transferred wafer is facilitated. Therefore, when the regenerated layer transferred wafer is reused for an SOI layer wafer in order to manufacture a bonded SOI wafer, oxygen precipitate nuclei or oxygen precipitates exist in an SOI layer.
In the SOI wafer having oxygen precipitates or oxygen precipitate nuclei in an SOI layer 111a as shown in FIG. 5(a), depending on the size of an oxygen precipitate nucleus or an oxygen precipitate 117, points where they exist become piercing defects 120 which pierces the SOI layer 111a as shown in FIG. 5(b). Therefore, when a non-illustrated natural oxide film on the surface of the bonded SOI wafer is cleaned by using a fluorinated acid (hydrofluoric acid) aqueous solution, the hydrofluoric acid aqueous solution etches an buried oxide film 113 between the SOI layer 111a and a support wafer 112 of the bonded wafer through each piercing defects 120 formed in the SOI layer 111a. As a result, as shown in FIG. 5(c), there is a problem that each HF defect 121 is generated.